It is known from patent application US 2007/0076182 A1 to provide an aerial image sensor in a laterally movable wafer stage. This terminology will be maintained also in conjunction with high aperture immersion lithography, although the image in this case is not generated in air, but in the immersion medium. It is also known, for example, from patent application US 2002/0041377 A1, to position such an aerial image sensor beside the actual wafer support. This known sensor operates in a slit-scan mode, and thus does not provide a simultaneous complete imaging.